The present invention relates to a method of and a device for controlling fuel injection in a multi-cylinder internal combustion engine having individual throttle valve controls for each cylinder and producing in response to predetermined crankshaft angles at least two fuel injections during a complete working cycle of a cylinder (suction, compression, explosion, discharge).
In a published European Patent Application No. 0069 386 (A2) a fuel injection system is described in which in the case of a four stroke engine fuel is injected into a first group of cylinders at a common time point whereas for a second group of cylinders fuel injection time is shifted by 180.degree. relative to the time point of the first group whereby two fuel injections related to the rotary speed of the crankshaft take place per a working cycle of each cylinder. In other words, a fuel injection occurs once per revolution of the crank shaft and inasmuch in the case of a four stroke engine the working cycle extends over two crankshaft rotations, the fuel injection occurs twice per 720.degree. position on the crankshaft.
In known fuel injection systems it has been assumed that in engines provided with individual throttle valve controls (a throttle valve per each cylinder) the pressure variations in intake pipe and hence the effective pressure differentials at the metering gap of respective fuel injection valves are very large with respect to the angle of crankshaft, when compared with conventional engines which have only a single throttle valve in the intake manifold. Such considerable intake pressure variations which periodically repeat after 720.degree. of the crankshaft are particularly strong in engines provided with four injection valves. In conventional fuel injection systems the end effect of such excessive intake pressure variations is the fact that in the usual simultaneous electrical actuation of all fuel injection valves, for example, the before-mentioned fuel injection per one revolution, different fuel injection doses are produced at respective metering gaps of the injection valves and these differences in the metered fuel quantity do not become equalized during a complete working cycle (0.degree. to 720.degree. of crankshaft position) of the cylinder. As a consequence, .lambda.-dispersions of unacceptable magnitude will occur between the individual cylinders. Such .lambda.-dispersions can amount up to 5% deviations in the preset mixture ratios and when considering the resulting degree of pollution and also when considering the possibility of highly accurate operation of digital fuel injection systems, such erratic values cannot be tolerated.
The cause of such excessive .lambda.-dispersions in multicylinder engines equipped with individual throttle valve controls for each cylinder is to be seen of course in the fact that a 720.degree. of crankshaft fuel is injected in all cylinders only twice. Therefore, in the case of uniform distribution and origin of movements of inlet and discharge valves in respective cylinders, non-uniform pressure differentials in the range of respective intake pipes are encountered when the injection time points are set at about 0.degree. (720.degree.) and 360.degree. of crankshaft at which angular positions, in the case of a four stroke engine, always only a single cylinder starts its suction stroke. In the case of a six cylinder engine, the injection time points are correspondingly shifted per 720.degree. range of the crankshaft in a periodic repetition for example at about 250.degree. and 310.degree. at which angular positions of the crankshaft also only a single cylinder initiates a suction stroke.
In prior art fuel injection systems attempts were made to avoid the disadvantages of excessive .lambda.-dispersion between the individual cylinders in such a manner that per 720.degree. of crankshaft (in a four stroke engine) altogether four injections take place whereby groups of cylinders are combined in such a manner that a uniform fuel distribution to individual cylinders will result. This prior art arrangement however has the disadvantage that already in a four stroke engine it is necessary to employ two end stages controlled at different times inasmuch the fuel injections occur in a 180.degree. spacings and in the case of a six cylinder engine there are accordingly needed three end stages to avoid non-symmetrical operation.